The invention relates to an application unit for directly or indirectly applying a liquid or pasty medium onto a moving material web, in particular consisting of paper or board.
Application units are known in which a dosing gap is formed between two lips as a dosing means operating as a free jet nozzle. This dosing gap is also called a color discharge gap. On account of the formation of a free jet of the liquid or pasty medium passing through the surrounding atmosphere, the designation "Fountain Jet Flow Applicator" (Jet Flow F) is also used for such application units. By means of the free jet, the liquid or pasty medium is directly or indirectly applied onto a moving material web.
In these application units, the liquid or pasty medium is generally supplied by a color distribution pipe arranged within a beam extending across the length of the application unit. The medium passes from the color distribution pipe through through-openings into a compensation space and flows from there via a feed channel to the dosing gap, from which the liquid or pasty medium is subsequently discharged in the form of a free jet. In the direct application of the medium, a material web passes the free jet nozzle and is directly impacted by the free jet of the liquid or pasty medium. In this case, for example, the material web can be guided on the surface of a roll. In indirect application of the medium, the free jet is initially applied onto a carrier surface, for example the surface of an application roll, in order to be transferred in a roll gap, through which the material passes, from the application roll onto the material web. The lip of the two lips forming the dosing gap which lies on the side of the dosing gap at which the application roll in the case of indirect application of the medium or the material web in the case of direct application of the medium moves towards the application unit is called the leading lip. Accordingly, the second lip which lies on the side of the dosing gap at which the application roll or the material web moves away from the application unit is called the trailing lip. The leading lip can have a concave deflecting surface.
Traditionally, in application units of the previously described type, an adjusting device is provided at the trailing lip by means of which the trailing lip can be adjusted in a zonewise manner along the length of the application unit with respect to its distance to the leading lip. As a result of this adjustment of the dosing gap, a certain transverse profile of the liquid or pasty medium applied onto the material web is achieved. Usually, downstream of the free jet applicator, there is a fine dosing device which doctors the applied liquid or pasty medium to the desired applied amount by means of a doctor blade element, for example a blade knife. Further, a collecting trough is arranged between the trailing lip of the application unit and the fine dosing device in order to collect excess liquid or pasty medium which has run off the application unit or the fine dosing device.
An exact adjustment of the dosing gap or the color discharge gap is very work- and time-intensive in conventional application units of the previously described kind and the profiling device for adjusting the gap is very expensive. In particular, when changing the type of liquid or pasty medium, the new adjustment of the dosing gap is very uneconomical. Additionally, on account of the finishing tolerances and possible assembly faults, an exact volumetric dosing across the entire web width can not be reliably guaranteed.